1. Field
The present invention relates to an optical transmitting apparatus that converts input data into an optical signal shaped into an RZ (Return-to-Zero) pulse and that synthesizes two optical signals of the same wavelength by setting their polarization states orthogonal to each other to transmit the synthesized optical signal, and to an optical communication system having the optical transmitting apparatus and an optical receiving apparatus to which an optical signal is transmitted.
2. Description of the Related Art
WDM (Wavelength Division Multiplex) has been known as a conventional technology in optical communication, by which technology a plurality of optical signals having different wavelengths are passed simultaneously through an optical fiber to transmit a large volume of information.
In WDM, generally, increasing the number of wavelengths or narrowing intervals between channels increases the volume of information that can be transmitted at once. Another method of increasing the volume of transmittable information has been known as a method of synthesizing two optical signals having the same wavelengths by setting their polarization states orthogonal to each other (see, e.g., Japanese Patent Application Laid-open No. H09-23193).
When an optical receiving apparatus that receives an optical signal receives such synthesized light, the optical receiving apparatus divides the synthesized light into two optical signals, and carries out a signal process on each divided optical signal to acquire data.
When the above synthesized light travels through an optical fiber in a long distance, polarization mode dispersion characteristic of the optical fiber occurs to break the orthogonal polarization relation between two optical signals, which causes both optical signals to exert effects each other. As a result, optical signals divided apart at the optical receiving apparatus have quality substantially inferior to an optical signal that is transmitted as a single optical signal.
When two optical signals are synthesized by setting their polarization states orthogonal to each other, therefore, the optical receiving apparatus has no other choice but to carry out the signal process on a heavily deteriorated optical signal. This leads to a problem of the occurrence of an error of data contents. Hence it has been required that even when the orthogonal relation between polarization states of two optical signals are broken due to polarization mode dispersion, effects exerted each other by both optical signals should be suppressed.